Researchers found that by using stem cells obtained from mature animals, they could ease the symptoms of muscular dystrophy, allowing previously crippled dogs to walk again. Sharon Hesterlee, the president of the Muscular Dystrophy Association, said the study allows for "cautious optimism" that similar treatment would be successful for humans.

The Nature study was the second announcement this week of a successful stem-cell treatment. On November 15, in a presentation for the American Heart Association, a team of Swiss doctors showed success in growing heart valves from stem cells obtained from amniotic fluid. These heart valves might be used to treat some of the 1 million babies who are born every year with congenital heart defects.

Neither of the two research breakthroughs involved stem cells that were taken from human embryos. To date, all of the most promising findings in the field of stem-cell therapy have come through the use of cells obtained from adults, or derived from amniotic fluid or placental tissues– sources that involve no significant ethical concerns.

In a survey of stem-cell research, published this week by the Family Research Council in Washington, DC, three analysts– Sarah Kleinfeld, William Saunders, and David Prentice– made the simple but trenchant observation that "embryonic stem-cell research has not yet yielded a single successful human treatment."